1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, and more particularly relates to a Mnxe2x80x94Zn ferrite suitable for use as various inductance elements, impedance elements for EMI countermeasures and the like, and to a production process thereof.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mnxe2x80x94Zn ferrite. Conventionally, this Mnxe2x80x94Zn ferrite usually has a basic component composition containing 52 to 55 mol % Fe2O3 on the average exceeding 50 mol %, which is the stoichiometric composition, 10 to 24 mol % ZnO and the remainder MnO. The Mnxe2x80x94Zn ferrite is usually produced by mixing respective material powders of Fe2O3ZnO and MnO in a prescribed ratio, subjecting mixed powders to respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a desired shape, then conducting sintering treatment at 1200 to 1400xc2x0 C. for 2 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is controlled to a low level by supplying nitrogen. The reason why the Mnxe2x80x94Zn ferrite is sintered in the reducing atmosphere is that Fe2+ is formed as the result of reducing a part of Fe3 +. This Fe2+ has positive crystal magnetic anisotropy and cancels out negative crystal magnetic anisotropy of Fe3 to thereby enhance soft magnetism.
Amount of the above-mentioned Fe2+ formed depends on relative partial pressures of oxygen in sintering and cooling after the sintering. Therefore, when the relative partial pressure of oxygen is improperly set, it becomes difficult to ensure excellent soft magnetic properties. Thus, conventionally, the following expression (1) has been experimentally established and the relative partial pressure of oxygen in sintering and in cooling after the sintering has been controlled strictly in accordance with this expression (1).
log Po2=xe2x88x9214540/(T+273)+bxe2x80x83xe2x80x83(1) 
where T is temperature (xc2x0 C.), Po2 is a relative partial pressure of oxygen, and b is a constant, which is usually set at 7 to 8. The fact that the constant b is 7 to 8 means that the relative partial pressure of oxygen in the sintering must be controlled in a narrow range, which makes the sintering treatment very troublesome, thereby increasing production costs.
Additionally, in recent years, with miniaturization and performance improvement of electronic equipments there is an increasing tendency that signals are processed at a higher frequency. Thus, a magnetic material having excellent magnetic properties even in a higher frequency region as well has been needed.
However, when the Mnxe2x80x94Zn ferrite is used as a magnetic core material, an eddy current flows in a higher frequency region applied resulting in a larger loss. Therefore, in order to extend an upper limit of the frequency at which the Mnxe2x80x94Zn ferrite can be used as a magnetic core material, an electrical resistivity thereof must be made as high as possible. However, since the above-mentioned general Mnxe2x80x94Zn ferrite contains Fe2O3 in an amount larger than 50 mol % which is the stoichiometric composition, a large amount of Fe 2+ ions are present, thereby making the transfer of electrons between the above-mentioned Fe3+ and Fe2+ ions easy. Thus, the electrical resistivity of the Mnxe2x80x94Zn ferrite is in the order of 1 xcexa9m or less. Accordingly, an applicable frequency is limited to about several hundreds kHz at highest, and in a frequency region exceeding the limit, permeability (initial permeability) is significantly lowered to take away properties of the soft magnetic material.
In order to increase an apparent resistance of the Mnxe2x80x94Zn ferrite, in some cases, CaO, SiO2 or the like is added as additive to impart a higher resistance to grain boundaries and at the same time the Mnxe2x80x94Zn ferrite is sintered at as low as about 1200xc2x0 C. to diminish the grain size from its usual dimension, about 20 xcexcm, to 5 xcexcm, which constitutes measures to increase the ratio of the grain boundary. However, even if such measures are adopted, it is difficult to obtain an electrical resistivity exceeding 1 xcexa9m order as the grain itself has a low resistance, and the above-mentioned measures fall short of a thorough solution.
Further, a Mnxe2x80x94Zn ferrite to which, for example, CaO, SiO2, SnO2 and TiO2 are added to obtain a higher resistance has been developed and is disclosed in Japanese Patent Application No. Hei 9-18092. However, the electrical resistivity of the Mnxe2x80x94Zn ferrite is as low as 0.3 to 2.0 xcexa9m, which is insufficient for application in a high frequency region. Further, a Mnxe2x80x94Zn ferrite containing 50 mol % or less Fe203 to which SnO2 or the like is added is disclosed in EPC 1,304,237. Although it is supposedly very difficult for Fe2+ to be formed when Fe2O3 content is 50 mol % or less, the Mnxe2x80x94Zn ferrite described in this EPC patent contains as much as 3 to 7 mol % Fe2+. Therefore, the electrical resistivity of the Mnxe2x80x94Zn ferrite in the EPC patent cannot exceed the electrical resistivity of a conventional general Mnxe2x80x94Zn ferrite.
On the other hand, a Mnxe2x80x94Zn based ferrite which contains less than 50 mol % Fe2O3 for a higher resistance has been developed for use as a core material for a deflecting yoke and is disclosed in Japanese Patent Laid-Open Nos. Hei 7-230909, Hei 10-208926, Hei 11-199235 and the like.
However, judging from the fact that the application thereof is a core material for a deflecting yoke and from examples of the invention described in each publication, the Mnxe2x80x94Zn based ferrites described in any of the above publications are ferrite materials intended to be applied in a frequency region of 64 to 100 kHz. It is described that reason for setting the Fe2O3 content to 50 mol % or less for obtaining a high resistance is to make it possible to wind a copper wire directly around a core for a deflecting yoke. Thus, those publications do not suggest the application of the Mnxe2x80x94Zn based ferrite in such a high frequency region as exceeding 1 MHz. All the Mnxe2x80x94Zn based ferrites have an initial permeability of about 1100 at 100 kHz, and excellent soft magnetic properties cannot be obtained by merely setting the Fe2O3 content to less than 50 mol % so as to obtain a high electrical resistivity.
Further, Japanese Patent Examined Application No. Sho 52-4753 discloses a Mnxe2x80x94Zn ferrite containing 50 mol % or less Fe2O3, to which 1.3 to 1.5 mol % CoO was added in order to decrease the temperature coefficient of initial permeability. This Mnxe2x80x94Zn ferrite also contains as low as 11 mol % ZnO and its relative partial pressure of oxygen at sintering and cooling is not strictly controlled. Thus, the initial permeability at 100 kHz is about 2000.
The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is therefore to provide a Mnxe2x80x94Zn ferrite which has a higher electrical resistivity than 1 xcexa9m order and at the same time high initial permeabilities of 4000 or more at 100 kHz and of 100 or more at 10 MHz, and a production process by which such a Mnxe2x80x94Zn ferrite can be obtained easily and inexpensively.
A Mnxe2x80x94Zn ferrite according to an aspect of the present invention to attain the above-mentioned object is characterized in that the main components include 44.0 to 49.8 mol % Fe2O3, 15.0 to 26.5 mol % ZnO, 0.1 to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn2O3 and the remainder MnO.
The present Mnxe2x80x94Zn ferrite may contain, in addition to the above-mentioned main components, at least one of 0.010 to 0.200 mass % V2O5, 0.005 to 0.100 mass % Bi2O3, 0.005 to 0.100 mass % In2O3, 0.005 to 0.100 mass % PbO, 0.001 to 0.100 mass % MoO3 and 0.001 to 0.100 mass % WO3 as additive.
Further, the present Mnxe2x80x94Zn ferrite has an initial permeability of 4000 or more at 100 kHz and 100 or more at 10 MHz at room temperature (25xc2x0 C.).
Still further, a production process according to the present invention to attain the above-mentioned object is characterized in that a mixed powder whose components are adjusted so as to obtain the composition of the above-mentioned Mnxe2x80x94Zn ferrite is pressed, then sintered and cooled, after the sintering, down to 500xc2x0 C. or lower in an oxygen atmosphere with a relative partial pressure of oxygen defined by an arbitrary value selected from a range of 6 to 10 as a constant b in the expression (1).
In a conventional general Mnxe2x80x94Zn ferrite, Fe2O3 content is more than 50 mol % that is the stoichiometric composition, as described above. Thus, in order to prevent this excessive Fe2O3 from getting precipitated as hematite, sintering and cooling had to be performed under a condition where a relative partial pressure of oxygen is reduced to a significantly lower level by flowing nitrogen, that is a condition obtained with the constant b set to 7 to 8. On the other hand, since the Mnxe2x80x94Zn ferrite of the present invention contains less than 50 mol % Fe2O3, hematite is hardly precipitated. Thus, even if a range of relative partial pressure of oxygen in sintering is somewhat increased, excellent magnetic properties can be obtained. Further, in a conventional Mnxe2x80x94Zn ferrite that contains more than 50 mol % Fe2O3, about 3.0 mol % Fe2+ exists. On the other hand, in the Mnxe2x80x94Zn ferrite of the present invention, Fe 2+ content is as little as 0.1 to 0.7 mol %. Accordingly, the electrical resistivity of the Mnxe2x80x94Zn ferrite of the present invention is very high. Therefore, an eddy current is not increased so much even in a high frequency region, and an excellent initial permeability can be obtained. However, if this Fe2O3 content is too small, the saturation magnetization is deteriorated. Thus, at least 44.0mol % Fe2O3 is needed.
ZnO as main component influences the Curie temperature and the saturation magnetization. Too small amount of ZnO reduces the initial permeability, but on the contrary, too large amount of ZnO lowers the saturation magnetization and the Curie temperature, so ZnO content is set to the above-mentioned range of 15.0 to 26.5 mol %.
Since Co2+ in CoO has a positive crystal magnetic anisotropy, CoO can cancel out a negative crystal magnetic anisotropy of Fe3+ even if Fe2+ having a positive crystal magnetic anisotropy exists only in a small amount. Further, Co2+ has an effect of reducing the loss in a high frequency region by generating an induction magnetic anisotropy. However, when CoO content is too small, the effect is small. On the contrary, when the CoO content is too large, the magnetic strain becomes large and the initial permeability is decreased. Thus, the CoO content is set to 0.1 to 3.0 mol %.
A manganese component in the above-mentioned ferrite exists as Mn2+ and Mn3+. However, since Mn3+ strains a crystal lattice, thereby significantly lowering the initial permeability, Mn2O3 content is set to 1.00 mol % or less. However, if the Mn2O3 content is too small, the electrical resistivity is significantly decreased. Thus, at least 0.02 mol % Mn2O3 is to be contained in the ferrite.
In the present invention, at least one of V2O5, Bi2O3, In2O3, PbO, MoO3 and WO3 can be contained as additive(s). All of these additives have an action of accelerating grain growth. The initial permeability in a comparatively lower frequency region depends on its grain size, so the initial permeability in a lower frequency region can be enhanced by allowing the above-mentioned additive(s) to be contained. However, if the content thereof is too small, the effects are small. On the contrary, if the content is too large, grains grow abnormally. Accordingly, it is desirable that V2O5 be set to 0.01 to 0.200 mass %, Bi2O3, In2O3 and PbO be respectively set to 0.005 to 0.100 mass %, and MoO3 and WO3 be respectively set to 0.001 to 0.100 mass %.
In the present invention, as described above, amount of Mn3+ is controlled by conducting sintering and cooling after the sintering in an atmosphere of a relative partial pressure of oxygen obtained by using an arbitrary value in a range of 6 to 10 as a constant b in the expression (1). When a value larger than 10 is selected as the constant b, the amount of Mn3+ in the ferrite becomes larger than 1 mol % whereby the initial permeability is rapidly lowered. Therefore, the amount of Mn3+ in the ferrite must be decreased to increase the initial permeability. Thus, it is desirable that a small value be selected as the constant b. However, when a value smaller than 6 is selected, amount of Fe 2+ is increased or amount of Mn3+ is decreased too much whereby the electrical resistivity is significantly lowered. Accordingly, the constant b is set to at least 6.
In production of the Mnxe2x80x94Zn ferrite, respective raw material powders of Fe2O3, ZnO, CoO, Mn2O3 and MnO, which are the main components, are previously weighed for a prescribed ratio, and mixed to obtain a mixed powder, and then this mixed powder is calcined and finely milled. Although temperatures for the calcination may be set slightly different from each other depending on target compositions, appropriate temperatures are selected from a range of 800 to 1000xc2x0 C. Further, a general-purpose ball mill can be used for the fine milling of the calcined powder. When V2O5 , Bi2O3, In2O3, PbO, MoO3 and WO3 are made to be contained as additives, proper amounts of powders of these additive are added to the fine milled powder and mixed with each other to obtain a mixture having a target composition. Then, in accordance with a usual ferrite production process, granulation and pressing are conducted, and sintering is then conducted at 1100 to 1400xc2x0 C. Incidentally, in the granulation a process of adding a binder such as polyvinyl alcohol, polyacrylamide, methyl cellulose, polyethylene oxide or glycerin can be used, and in the pressing a process of applying pressure of, for example, 80 MPa or more can be used.
In the above-mentioned sintering and cooling after the sintering, a relative partial pressure of oxygen is controlled by flowing inert gas such as nitrogen gas or the like into a sintering furnace. In this case, as the constant b in the expression (1), an arbitrary value can be selected from a range of 6 to 10. Thus, the constant b has a very wide allowance as compared to the constant b (7 to 8) selected in a case where a conventional general Mnxe2x80x94Zn ferrite containing more than 50 mol % Fe2O3 is sintered, and the relative partial pressure of oxygen can be easily controlled. Further, in this case, the cooling after the sintering needs to be performed in accordance with the above-mentioned expression (1) only until the temperature gets down to 500xc2x0 C. because the reaction of oxidation or reduction at a temperature lower than 500xc2x0 C. can be ignored irrespective of the relative partial pressure of oxygen.